Problems of nuclear radiation shielding for both personnel and certain instruments arise in a wide variety of activities such as power reactors, industrial processing and non-destructive testing. The problem also is found in such fields as medicine, space technology, oceanography and in fundamental studies involving basic experimental research.
Shielding materials are required to meet increasingly more diverse performance criteria as the applications of nuclear radiation expand to new areas and make significant improvements in old areas. These applications, as a group, include all types of radiations but, individually, each has its own special shielding requirement.
A large number of radiation shielding problems involve neutrons and, in this area especially, the hydrogen atom as a component of the shield has a particular value, because the hydrogen atom is the most effective of all atoms in moderating neutrons by elastic scatter. This process occurs without the production of gamma photons, which are produced by neutron energy lost by inelastic scatter when a neutron strikes a massive atom with a high atomic number. For this reason, hydrogenous materials, especially those with a high hydrogen atom concentration have been widely employed to attenuate neutrons from many sources, especially those produced in nuclear reactors, particle accelerators, and certain radioisotopic materials.
With only a few exceptions, materials containing a high concentration of hydrogen atoms are combustible in air. Non-combustible hydrogenous materials generally contain a much lower concentration of hydrogen atoms, and consequently do not provide the desired efficiency of neutron attenuation. Water is the obvious exception, but this substance has the great disadvantage of not being self-supporting under usual operating conditions of temperature and pressure, and complications arise from container problems when containers are used.